3rd Generation Partnership Project (3GPP) networks, beginning in Release 9, support the use of enhanced-uplink dedicated channels (E-DCH) in the forward access channel (CELL_FACH) cell state and Idle Mode, in addition to the dedicated channel (CELL_DCH) cell state. Previously, wireless communication devices (WCDs) had to use a random-access channel (RACH) while in the CELL_FACH cell state or in Idle Mode.
When a WCD in the CELL_FACH state has data to transmit in the uplink (UL), the WCD may request a common E-DCH resource from the network. The network allocates a common E-DCH resource to the WCD. Then, the WCD can use the allocated common E-DCH resource to transmit its UL data. In some configurations, when the WCD has finished transmitting all of its UL data (i.e., when the WCD has emptied its transmit buffer), the WCD waits a certain amount of time (i.e., the E-DCH transmission continuation back-off timer) and then sends a Media Access Control (MAC) protocol data unit (PDU) containing scheduling information (SI) with the Total E-DCH Buffer Status (TEBS) set to zero. In response to receiving a Layer 1 ACK sent in response to the MAC-PDU containing the SI, the WCD releases the common E-DCH resource that the network had allocated to the WCD.
When a WCD is commanded to transition from the CELL_FACH state to the CELL_DCH state (e.g., the WCD receives a Radio Bearer (RB) Reconfiguration message) and the WCD does not have a common E-DCH resource, then the WCD must synchronize with the network. This synchronization may take several hundreds of milliseconds, during which time data may not be transmitted. If the WCD instead has a common E-DCH resource, according to 3GPP TS 25.331, after receiving an RB Reconfiguration message instructing the WCD to transition to CELL_DCH cell state, the WCD has at most 100 ms to apply the new configuration and must inform the network within 110 ms. That is because if the WCD has a common E-DCH resource during reconfiguration to the CELL_DCH state, the WCD may not need to synchronize with the network and seamless transition may be possible. That is, the reconfiguration procedure from CELL_FACH state to CELL_DCH will be speeded up if the WCD has a common E-DCH resource during the reconfiguration.
One way to ensure that the WCD has a common E-DCH resource when a reconfiguration occurs involves transmitting a concrete HS-SCCH order from the network to the WCD, a feature introduced in 3GPP Release 11, called NodeB triggered HS-DPCCH transmission. The problem with this method is that it depends on WCDs implementing the 3GPP Release 11 standard, and so would not work with legacy WCDs implementing 3GPP Release 8, 9, or 10.
Another method relies on sending Radio Link Control (RLC) Acknowledged Mode (AM) data in the downlink, with the polling information set, so as to trigger an RLC Acknowledgment in the uplink. But in this method, the Acknowledgement may be delayed by the parameter Timer_Status_Prohibit (3GPP 25.322 TS), which controls how much time has to elapse between two consecutive Acknowledgements. Additionally, the method does not prevent the WCD from releasing the E-DCH resource before reconfiguration finishes. Further, there is a risk in this method of the WCD missing the RLC AM downlink data, which would trigger retransmission of the RLC AM downlink data, degrading the overall network performance and possibly causing a mismatch between the protocol variables in the transmitter and receiver. This method also affects RLC protocol variables.
Another method relies on increasing the duration of the E-DCH transmission continuation back-off timer. Increasing the duration of this timer, however, may not be a viable solution since the timer is broadcast and used by all the WCDs that have acquired a common E-DCH resource for DCCH/DTCH transmission. Moreover, a large value of the timer would result in decreased network capacity and increased UL interference (the WCD keeps on transmitting the UL DPCCH until the resources are released).
Accordingly, there is a need for a method to ensure seamless transition from CELL_FACH to CELL_DCH cell state in a wireless communication device, in a reliable and robust way, that does not interfere with RLC protocol variables and is backwards-compatible with legacy WCDs.